What is reactor dosimetry?

 

The RPV (Reactor Pressure Vessel) of a nuclear reactor is a key issue in its safety assessment. This vessel is a primary loop barrier and contains the nuclear fuel. During reactor operation the RPV is subject to severe conditions (gamma and neutron radiation, temperature gradients), which gradually degrades the vessel.

The main damaging mechanism is the material embrittlement due to its exposure to (fast) neutrons. The embrittlement of the RPV is the limiting factor for the lifetime of the reactor. Therefore all modern nuclear reactors are equipped with a set of RPV surveillance capsules containing representative steel specimens (to investigate the material degradation) and neutron dosimeters (to determine the neutron exposure responsible for the degradation). The capsules are introduced close to the RPV before the reactor start and dismantled and analysed on a regular basis during its lifetime.

Reactor dosimetry thus aims at determining the neutron fluences that were present at the RPV during the reactor operational lifetime. This information can be obtained from the measured activity of the irradiated neutron dosimeters and the detailed RPV irradiation history (including neutron spectrum calculations).

An important activity in the field of reactor safety is the irradiation of materials. For existing reactors with an inadequate RPV surveillance programme so-called advanced material testing irradiations in high-flux reactors are sometimes required. Also, new types of material or nuclear instrumentation need to be tested in high neutron fluxes for its possible application in next generation reactors. Although neutronic calculations are mostly performed for such experiments, they should always be equipped with neutron dosimeters for an accurate determination of the neutron flux or fluence (time integrated flux). 

 


Drawing of a typical reactor pressure vessel